SYK (Spleen Tyrosine Kinase) is an intracellular tyrosine kinase that is involved in coupling activated immunoreceptors to signal downstream events that mediate diverse cellular responses, including proliferation, differentiation and phagocytosis.
The receptors in which SYK performs an important function in signal transduction include for example the receptors for IgE (FcεRI) and IgG (FcγR1) on mast cells and B cells, the B-cell receptor (BCR) and the T-cell receptor (TCR) on B- and T-cells, the ICAM1 receptor (ICAM1 R) on epithelial cells of the respiratory tract, the DAP12-receptor on natural killer cells, dendritic cells and osteoclasts, the dectin 1-receptor on a subpopulation of T-helper cells (Th-17 cells), as well as the integrin receptors for β1-, β2- and β3-integrins on neutrophils, monocytes and macrophages (Ruzza et al., Expert Opin. Ther. Patents, 2009, 19 (10), 1361-1376; Ulanova et al., Expert Opin. Ther., Target., 2005, 9 (5), 901-921; Wang et al., J. Immunol., 2006, 177, 6859-6870; Slack et el., European J. Immunol., 2007, 37, 1600-1612).
Dysregulation and/or misregulation of different signal transduction pathways of SYK in different cell types have been implicated in numerous diseases and disorders e.g., allergic rhinitis, asthma, autoimmune diseases, rheumatoid arthritis (RA), osteopenia, osteoporosis, COPD and various leukemia and lymphomas. The inhibition of SYK activity by the present invention may offer a therapeutic option for treatment of many diseases associated with SYK activity.
Rheumatoid arthritis (RA) is an auto-immune disease characterized by inflammation of articular joints leading to debilitating destruction of bone and cartilage.
Studies using cells from SYK knocked-out mice displayed characteristic phenotypes by blocking in B cell development (M. Turner et al., Nature, 1995, 378, 298-302; Cheng et al., Nature, 1995, 378, 303-306). These studies and elsewhere demonstrate that SYK is required for the differentiation and activation of B cells. Therefore, inhibition of SYK activity in RA patients is likely to block B cell function and hence to reduce rheumatoid factor production. In addition to the role of SYK in B cell function, the requirement for SYK activity in Fc receptor (FcR) signaling is relevant to treatment of RA. FcR activation by immune complexes in RA has been suggested to contribute to the release of multiple pro-inflammatory mediators.
It was demonstrated that targeting B cell function by antibody rituximab, a B cell depleting antibody is an appropriate therapeutic strategy to treat auto-immune diseases such as RA (Edwards et al., New Eng. J. Med., 2004, 350 (25), 2572-2581). Furthermore, genetic deficiency of SYK in the hematopoietic compartment completely blocked the development of all macroscopic and microscopic signs of arthritis in autoantibody-induced arthritis mice model. In addition, it was demonstrated that the SYK−/− mutation prevented the appearance of periarticular bone erosions. Finally, SYK−/− bone marrow chimeras were completely protected from arthritis-induced loss of articular function (Jakus et al., Arthritis Rheum., 2010, 62 (7), 1899-1910).
SYK inhibitors may also be useful in treatment of systemic lupus erythematosus (SLE) which is a chronic inflammatory autoimmune disease and can affect several organs and systems. Several studies have shown that T-cell receptor (TCR) signaling is modified in patients suffering from SLE (Tsokos G C., N Engl J Med, 2011, 365, 2110-2121). Instead of transmitting signals through TCR to CD3ζ and Zap70, an alternative pathway comes into play involving FcRγ and SYK ([Krishnan et l., J Immunol., 2008, 181, 8145-8152). FcRγ is homologous in shape and function to CD3 and takes its place in SLE T cells and associates with Syk. This alternative FcRγ/Syk duet is 100 times enzymatically more potent than the canonical CD3ζ/Zap70. As a result, following activation, SLE T cells exhibit higher intracytoplasmic calcium flux and cytosolic protein tyrosine phosphorylation (Nambiar et al., J. Immunol., 2003, 170, 2871-2876).
SYK inhibitors may also be useful in cancer therapy, specifically heme malignancies, particularly Non-Hodgkin's Lymphomas including follicular (FL), mantle cell, Burkitt and diffuse large B cell (DLBCL) lymphomas. SYK is found to be dysregulated by overexpression and/or constitutively activation in a variety of primary B-lymphoma tumors and in B-lymphoma cell lines. Through the PI3K/AKT pathway, the PLD pathway and AKT independent signalling, SYK is known to activate mTOR (mammalian target of rapamycin) which in turn increases B-cell survival and proliferation Inhibition of SYK in vitro results in decreased mTOR activation and a reduction of clonicity in FL cells and diffuse large B cell lymphoma (DLBCL) (Lesux L. et al., Blood, 2006, 108(13), 4156-4162 and Guruajan M. et al., J. Immun., 2007, 178, 111-121).
SYK inhibitors may also be useful in the treatment of asthma and rhinitis. Allergic rhinitis and asthma are diseases associated with hypersensitivity reactions and inflammatory events involving a multitude of cell types including mast cells, eosinophils, T cells and dendritic cells. SYK is positioned in transducing the downstream cellular signals associated with cross-linking FcεR1 and FcγR1 receptors. Following exposure to allergen, high affinity immunoglobulin receptors for IgE (FcεR1) and IgG (FcγR1) become cross-linked and activate downstream processes in mast cells and other cell types leading to release of pro-inflammatory mediators and airway spasmogens. In the mast cell, for example, IgE receptor cross-linking by allergen leads to release of mediators including histamine from preformed granules, as well as the synthesis and release of newly synthesized lipid mediators including prostaglandins and leukotrienes, which lead inflammatory events.
SYK inhibitors may also be useful in the treatment of urticaria triggered by allergic reactions but many cases have an unclear etiology. Acute and chronic urticaria are common skin diseases. There are many pathological similarities in chronic urticaria patients with allergen-induced mast and basophil cell degranulation reactions via IgE activation. Around 40% of chronic spontaneous urticaria patients contain serum IgG auto-antibodies targeting IgE or the FcεR and these are thought to drive the histamine and other mediator release via mast and basophil degranulation. SYK inhibitors would inhibit the signaling response post IgE medicated FcεR activation and inhibit the mediator release known to be involved in chronic pruritis in multiple diseases.
An inhibitor of the SYK kinase activity could also be used therapeutically in treating chronic obstructive pulmonary disease (COPD) caused by microbes and allegens. COPD is characterised by a successive deterioration in lung function and chronic inflammation of the airways, which is initiated and produced by noxious substances of all kinds and contributes to the maintenance of the course of the disease. At a cellular level, in COPD there is in particular a multiplication of T lymphocytes, neutrophils, granulocytes and macrophages. An increase in the number of CD8-positive lymphocytes is known to be directly connected with the impairment of lung function. Another characteristic of COPD are acute deteriorations in lung function (exacerbations), characterised by viral (e.g. Rhinovirus), or bacterial (e.g. Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis) infections. An inhibitor of the SYK kinase activity could also be used therapeutically in acute lung deteriorations caused by Rhinoviruses.
WO03/057695A1 (Boehringer Ingelheim Pharmaceuticals, Inc.) describes novel 1,6-naphthyridines that have SYK inhibitory activity. Three more recent patent applications, WO2010/015518A2, WO2010/015520A1 and WO2011/092128A1 (Boehringer Ingelheim International GmbH) disclose compounds having SYK inhibitory activity.
WO04/035604A2 (Millennium Pharmaceuticals, Inc.) discloses the structural co-ordinates of the human SYK protein.
WO 2011/134971 A1 (Glaxo Group Ltd.) discloses 7-(1H-pyrazol-4-yl)-1,6-naphthyridine compounds as SYK inhibitors.
WO 2011/144585 A1 (F. Hoffmann-La Roche A G) discloses the pyrrolo[2,3-B]pyrazine-7-carboxamide derivatives and their use as JAK and SYK inhibitors.
There remains, however, a need to identify further compounds which are inhibitors of spleen tyrosine kinase (SYK).